I will tell you this, I have a 87 f-150 with the inline 6. I dont have a muffler, I just have a y-pipe in place for it and ran duals out the back, and it is still quiet. Maybe removing the cats, or putting a hi-flow on there will help sound alittle. But it is extremely hard to get a nice sound out of these engines. But i did cut off my exhaust..and i had like 2 feet of pipe coming out of the cat and just ending there, that had a nice rumble to it. But then I added some pipe to it, and the rumble just disappeared. Maybe some other people had better luck getting a nice sound out of these engines that can help you.

Edit: This was suppose to be a reply on the thread on the exhaust on the 94 4.9 liter. I dont know what happen.